I have been spending a lot of time learning about the mechanics of photography now that I have my new G7 camera. As with a lot of hobbies, there’s technical knowledge — how stuff works — and practical knowledge — what you do with that stuff. One thing that has been throwing me off on both accounts is the difference between full-frame, APS-C, and micro 4/3 cameras and how this relates to lenses.

The G7 is a micro 4/3 camera (referred to as a 4/3 going forward). 4/3 is a way of indicating how large the sensor is in relation to a full-frame sensor. As I don’t have an APS-C camera, I’m not going to talk much about it, but know that APS-C sensors sit between full-frame and 4/3, but closer to the 4/3 side of the spectrum.

The sensor is the part of the camera which processes the light into a photo.

As the light enters the lens — the tube attached to the front of the camera — the actual lens(es) within that tube “push the light around”. This is physics and is way beyond my point and understanding, but know that the way these lenses push the light around results in a point within the lens where the light converges.

When we measure the distance between the point of convergence and the sensor of the camera, we have the focal length. Focal length is usually written in terms of “millimeters” or “mm”. To put it in incomplete, unscientific, don’t-@-me-in-the-comments terms, the focal length value of a lens can tell you whether or not the lens attached to the camera will give you a wide field of view (<30mm) or can bring far away objects “closer” (>70mm).

I’ve been looking at other lenses for the G7, and every time I open a browser to do research, I run into people shouting about the concept of “crop factor” in relation to 4/3 and APS-C cameras. It’s pretty plain to understand that an APS-C sensor is smaller than a full-frame sensor, and a 4/3 sensor is smaller than an APS-C sensor, but what the hell does that really mean, and what does it mean for lenses?

At its most basic, a smaller sensor means two things. First, less light can be captured by the sensor. As you can see in the image above, a full-frame camera has a crop factor of 1.0. My G7 — a 4/3 sensor camera — has a crop factor of 2.0, therefor my G7 collects 1/2 as much light as a full-frame camera. This has a big impact on my ability to take night and low-light photos, depth of field, bokeh, and how I have to think about lighting situations in general compared to how I’d think about it if using a camera with a larger sensor.

The second thing about a smaller sensor is that the field of view is going to be smaller than that of a full-frame sensor. As mentioned above, the choice of lens also matters in field of view. A smaller focal length (say 25mm) is going to capture more than a larger focal length (say 200mm) when aimed at the same subject. If we can switch lenses to modify the field of view, how does that jive with sensor size?

This is where the pain comes in for new sub-full-frame camera owners when looking at lenses. At some point a new camera owner is going to hear about “crop factor” and how we’ll need to multiply the focal length of our 4/3 lenses in order to get the equivalent full-frame focal length. While technically true, it’s a complete disservice when helping out a new photographer with lenses for a non-full-frame camera.

Ultimately, a focal length is the focal length as stated. 50mm is always going to be 50mm regardless of whether its a lens designed for a 4/3 camera or a lens designed for a full-frame camera. When folks start bringing crop factor math into the equation, they are talking about how to get the same field of view from your 4/3 lens that you would get from a full-frame sensor lens.

If we were to take a photo of the same landscape with four different cameras — full-frame, APS-C, 4/3, and a point-and-shoot — with the lenses of the same focal length (say 30mm, but the image above doesn’t specify) designed for that size camera, this is what each of the resulting images would look like. Note that these results are not a factor of the lens or focal length, but a factor of the sensor of the camera. Our 4/3 camera is going to capture the exact same image when using a native 30mm lens as the full-frame camera using a native 30mm lens, but the 4/3 image will not have the same field of view as the full-frame image.

Ultimately, describing crop factor math to a new 4/3 or APS-C photographer is not very helpful. Most of the posts and videos I have seen on the subject only result in confusion and people believing that the focal lengths printed on a lens designed for a 4/3 camera is not the actual focal length. It is! But a 50mm prime built for a 4/3 camera isn’t going to give you the same field of view as you’d get from a 50mm prime lens built for a full-frame camera. If that kind of thing is important to you, then crop factor math will be important. It’s just not important to tell someone who is looking to buy a lens specifically made for their non-full-frame camera. Crop factor math also comes into play if you are using a lens made for one size of camera on a smaller size of camera with an adapter. At this point you’re also going to run into issues with f-stop values, as the actual f-stop will no longer be the realized f-stop on account of — once again — how much light your sensor can process.

Coming to this understanding makes me feel a lot better about the choices I have with my 4/3 sensor camera. It’s said that a 50mm lens “mimics the human eye”, but really that’s when talking about 50mm full-frame lenses. In order to get that kind of field of view on my camera I would need a 25mm 4/3 lens if “mimicking the human eye” is the important part of having that lens. Ultimately what I lose out on with a 4/3 50mm lens is the field of view, but knowing this I can simply compensate by only taking into consideration what my camera is capable of. 50mm on my camera is still 50mm, and 200mm will be 200m, and 14mm will still be 14mm. I don’t have to think about any of this in terms of “full-frame sensor equivalence” because I have never owned a full-frame sensor camera and don’t know a damn bit of difference. I’ll simply shoot what I see with my G7, and will make that work with the lenses I eventually acquire.

For a really good explanation with some pretty inventive examples, check out this video.